As is known in the art, an optical amplifier is a device that increases the amplitude of an input optical signal fed thereto. If the optical signal at the input to such an amplifier is monochromatic, the output will also be monochromatic, with the same frequency. A conventional optical amplifier comprises a gain medium, such as a single mode glass fiber having a core doped with a rare earth material, connected to a WDM coupler which provides low insertion loss at both the input signal and pump wavelengths. The input signal is provided, via the coupler, to the medium. Excitation occurs through optical pumping from the pumping source. Pump energy that is within the absorption band of the rare earth dopant is combined with the optical input signal within the coupler, and applied to the medium. The pump energy is absorbed by the gain medium, and the input signal is amplified by stimulated emission from the gain medium.
Such amplifiers are typically used in a variety of applications including, but not limited to, amplification of weak optical pulses such as those that have traveled through a long length of optical fiber in communication systems. Optical amplification can take place in a variety of materials including those materials, such as silica, from which optical fibers are formed. Thus, a signal propagating on a silica-based optical fiber can be introduced to a silica-based optical fiber amplifier, and amplified by coupling pump energy into the amplifier gain medium.
Fiber amplifiers are generally constructed by adding impurities to (i.e. "doping") an optical fiber. For a silica-based fiber, such dopants include the elements erbium and ytterbium. For example, one type of fiber amplifier referred to as an erbium (Er) amplifier typically includes a silica fiber having a single-mode core doped with erbium ions (conventionally denoted as Er.sup.3+). It is well known that an erbium optical fiber amplifier operating in its standard so-called three level mode is capable, when pumped at a wavelength of 980 nanometers (nm), of amplifying optical signals having a wavelength of approximately 1550 nanometers (nm). Likewise, an amplifier having a silica-based fiber "co-doped" with erbium and ytterbium shows excellent amplification of a 1550 nm optical signal when pumped with a wavelength from about 980 nm to about 1100 nm. A particularly useful pump wavelength is 1060 nm because of the availability of high power solid state laser sources at about 1060 nm. Since 1550 nm is approximately the lowest loss wavelength of conventional single-mode glass fibers, these amplifiers are well-suited for inclusion in fiber systems that propagate optical signals in the wavelength vicinity of 1550 nm.
In certain applications, it is desirable to amplify, or generate, a polarized signal using a fiber amplifier or laser, respectively, while maintaining polarization of the optical signal at the output. While a high-birefringence, polarization maintaining fiber may be used, this fiber is difficult to manufacture and generally unavailable as a rare earth doped fiber. One method of providing polarization-maintaining optical amplification is disclosed in U.S. Pat. No. 5,303,314 to Duling III et al. This patent discloses the use of an amplifier having non-polarization maintaining fiber. A linearly polarized optical signal is directed through the amplifier fiber to a Faraday rotator mirror, where it is reflected, and the orientation of its polarization is shifted by 90.degree.. While passing back through the amplifier fiber, any polarization changes caused by the fiber during the initial pass are undone. The reflected signal is thereafter directed to a polarization beamsplitter, where the polarization shift provided by the Faraday rotator mirror allows the returning signal to be separated from the input signal.